Sawn lumber in standard dimensions is the major construction material used in framing homes and many commercial structures. The available old growth forests that once provided most of this lumber have now largely been cut. Most of the lumber produced today is from much smaller trees from natural second growth forests and, increasingly, from tree plantations. Intensively managed plantation forests stocked with genetically improved trees are now being harvested on cycles that vary from about 25 to 40 years in the pine region of the southeastern and south central United States and about 40 to 60 years in the Douglas-fir region of the Pacific Northwest. Similar short harvesting cycles are also being used in many other parts of the world where managed forests are important to the economy. Plantation thinnings, trees from 15 to 25 years old, are also a source of small saw logs.
Whereas old growth trees were typically between two to six feet in diameter at the base (0.6 m to 1.8 m), plantation trees are much smaller. Rarely are they more than two feet (0.6 m) at the base and usually they are considerably less than that. One might consider as an example a typical 35 year old North Carolina loblolly pine plantation tree on a good growing site. The site would have been initially planted to about 900 trees per hectare (400 per acre) and thinned to half that number by 15 years. A plot would often have been fertilized one or more times during its growth cycle, usually at ages 15, 20 and 25 years. A typical 35 year old tree at harvest would be about 40 cm (16 in) diameter at the base and 15 cm (6 in) at a height of 20 m (66 ft). Trees from the Douglas-fir region would normally be allowed to grow somewhat larger before harvest.
American construction lumber, so-called "dimension lumber", is nominally 2 inches (actually 11/2 inches (38 mm)) in thickness and varies in 2 inch (51 mm) width increments from 31/2 inches to 111/4 inches (89 mm to 285 mm), measured at about 12% moisture content. Lengths typically begin at 8 feet (2.43 m) and increase in 2 foot (0.61 m) intervals up to 20 ft (6.10 m). Unfortunately, when using logs from plantation trees it is now no longer possible to produce the larger and/or longer sizes and grades in the same quantities as in the past.
There is another problem with plantation wood that is not as generally recognized as are the size limitations. Typically, in plantation wood the average wood density is lower than old growth wood. This, in turn, affects strength and stiffness. Strength in flexure, otherwise termed modulus of rupture, and especially the stiffness measured as modulus of elasticity in flexure, may be somewhat lower and possibly more variable than old growth wood. This is a problem for members used in a bending situation and it can be for those members used in compression; e.g. longer wall studs. Typical of bending uses are floor joists, truss members, and headers over wide windows and doors, such as garage doors.
The trunk of a tree may be visualized as a stack of hollow cones of ever increasing length and base diameter and ever decreasing included angle. Each cone depicts a single annual growth increment that proceeds from the top of the tree to the base. Until after about 15 annual growth rings have been formed, wood at any height above the base in the southern pine species and Douglas-fir has juvenile properties characterized by relatively wide growth rings and relatively low density. For loblolly pine trees older than about 15 years (about 20 years for Douglas-fir), in any given growth year the wood in the upper part of the conical growth increment still has juvenile characteristics while the wood at the base of the same annual growth increment is of a denser more mature type. Thus, a tree might be visualized as having a cylinder of juvenile-type wood about 15 growth rings wide running the entire length to the point of its minimum diameter useable as a saw log. If a saw log taken from the top of the tree has only about fifteen growth rings or less it will consist almost entirely of relatively low density juvenile wood. Beyond that age, wood of mature characteristics will be found only in the outer portions of the tree. One of the characteristics of the more mature wood is a significantly higher density with, generally, a higher ratio of late wood to early wood and narrower ring spacing than that of the juvenile wood.
As growth progresses the core portion of the tree becomes infused with resinous and other materials and ceases to be a physiologically functioning part of the plant. The function of this resinous heartwood, as it is called, is essentially that of structural support. The change to heartwood does not significantly affect strength, however. The juvenile characteristics of the wood remains unchanged.
Since loblolly pine (Pinus taeda L.) and its closely related southern pines are particularly important timber species they will be used in the following discussion as a non-limiting example of trees in general. Along any given radius density increases approximately linearly from the pith to about 15 years of age beyond which time there is little further increase. Douglas-fir has a somewhat different pattern. Density will normally decrease for eight to ten rings outward from the pith then gradually increase for fifty rings or more.
A frequently used unit related to density is specific gravity measured as oven dry weight/green volume. For loblolly pine, near the base of the tree specific gravity of the first several growth rings surrounding the pith will typically range around 0.38. By about age 20 the wood being formed near the bark at the same height will have a specific gravity of about 0.51-0.56. Density even of the outer mature wood portion of the tree varies longitudinally along the tree, being generally lower in the upper portions. Density of woods has been shown to correlate directly with stiffness, measured as modulus of elasticity in flexure.
R. A. Megraw, in Wood Quality Factors in Loblolly Pine, TAPPI Press, Atlanta, Ga. (1985) discusses in depth the influence of tree age, location in the tree, and cultural practice on wood specific gravity, and fiber length. He observes as noted above that inner growth rings (out to about 15 years) are wider with lower specific gravity while those beyond that point are narrower and of higher specific gravity Further, the specific gravity of the outer rings decreases 10-15% between the base and about 5 m in height and at a slower rate to heights of 15 m or more. These factors all contribute to variability in strength. This variability has not been seriously taken into account in the manufacture of lumber products. Current sawmill procedures make no attempt to take advantage of these inherent differences in density. The general assumption appears to have been that this was a factor which was not subject to any control.
Solid sawn wide dimension lumber is not without its own significant drawbacks. In particular, inconsistency in dry dimensions and strength properties and poor availability of long lengths are major deficiencies. Variability in grain orientation and differences and changes in moisture content result in dimensional instability before and after installation. Inconsistent width from piece to piece results in poor conformation of sheathing or subfloor. In the case of subflooring this is a major contributor to the cause of annoying squeaks as people walk on the floor.
Many approaches have been taken to engineer structural grade wood products to take the place of the larger and/or longer lumber sizes now in short supply. One successful approach is based on adhesively bonding a number of plies of rotary cut veneer. Unlike typical plywood products, the grain direction of all the plies is normally in the same direction. In one way of producing this product wide panels of appropriate thickness are ripped into pieces of standard dimension lumber width then finger jointed to the desired length. Other processes start with relatively narrower veneer sheets which can be butted end-to-end and continuously bonded to make units of almost any desired length, width, and thickness. The butt joints of adjoining plies are preferably staggered to prevent introducing points of weakness. This so-called laminated veneer lumber (LVL) has been in commercial production and use for a number of years, often as the tension members of trusses; e.g., as seen in Troutner, U.S. Pat. No. 3,813,842. It has the advantage that defects, particularly knots, do not run entirely through the piece as they do in sawn wood. This generally allows a higher stress rating for a LVL member of any given cross sectional dimensions. However, LVL initially requires very high grade "peeler" logs and high adhesive usage, both of which have an adverse effect on cost. Other exemplary products of this type are described by Peter Koch, Beams from bolt-wood: a feasibility study, Forest Products Journal, 14: 497-500 (1964) and by E. L. Schaffer et al., Feasibility of producing a high yield laminated structural product, U.S.D.A. Forest Research Paper FPL 175 (1972).
Many combinations of veneer, solid sawn wood, and reconstituted wood such as engineered strandboard or flakeboard have also been explored for use as structural lumber products. Lambuth, in U.S. Pat. No. 4,355,754, shows a structural member in the form of an I-beam using a plywood web with solid sawn flange members. When used as a joist, this is presumably substitutable for sawn lumber of the same cross sectional dimensions. The web is friction fit and glued into tapered slots in the flange pieces. Other very similar constructions use composite wood strips such as oriented strandboard or flakeboard as the web member.
Barnes, in U.S. Pat. No. 5,096,765, notes the importance of stiffness (modulus of elasticity in flexure) (MOE) in lumber products. The product described uses splinters or strands of sliced veneer from 0.005-0.1 inch (0.13-2.5 mm) thick, at least 0.25 inches (6.4 mm) wide and at least 8 inches (203 mm) long. These must be free of any surface or internal damage and have their grain direction within 10.degree. of the longitudinal axis of the product. After addition of adhesive the product is pressed to have "an MOE equivalent to a composite wood product having a MOE of at least 2.3 mm psi [1.59.times.10.sup.7 kpa] at product (sic) a wood content density of 35 lbs/cubic foot".
In the above patent the inventor refers to his earlier U.S. Pat. No. 4,061,819 which teaches that the strength of wood composite products is density dependent; i.e., ". . . the higher [the] density generally the higher the strength of the product for the same starting materials". The earlier patent describes a very similar lumber-like product to the above having a modulus of elasticity approaching or reaching the MOE of clear Douglas-fir at various densities. Products similar to those described in the Barnes patents are now commercially available. However, the very high adhesive usage they require has a significant negative impact on cost of the products. Also, the strandwood products have significantly higher density than sawn lumber and are heavier to handle and more expensive to ship.
Many other patents teach the manufacture of clear wood members by various combinations of sawing and edge, end, and/or face gluing. Exemplary of these are U.S. Pat. No. 1,594,889 to Loetscher, U.S. Pat. No. 1,638,262 to Neumann, U.S. Pat. No. 2,942,635 to Horne, U.S. Pat. No. 5,034,259 to Barker, and U.S. Pat. No. 5,050,653 to Brown. Other workers have explored surface densification for various purposes, Exemplary of these are U.S. Pat. No. 3,591,448 to Elmendorf and U.S. Pat. No. 4,355,754 to Lund et al. Most of the products noted above have not found significant success for one or more reasons. There are exceptions, however. Laminated veneer lumber and edge and end glued pieces reassembled to produce clear boards or for use as door cores have been in commercial use for many years. Composite I-beams similar to those described in the Lambuth patent are now also widely available. One such product family manufactured by Trus Joist MacMillan, Boise, Id., is typical of the products which appear to have become an industry standard.
The composite I-beams have found considerable acceptance in the building industry where long spans, consistent dimensions and known and dependable strength properties are required. However, they are not without their drawbacks. Their performance under common residential dynamic loads is not as good as solid sawn construction, due primarily to a lack of mass. As a result most builders use I-joists at a shorter than suggested span or at a reduced spacing. They cannot entirely be used as a replacement for sawn lumber. For example, they need reinforcing blocking to fill out the sides of the web to fill width at many loading points. Their cross section essentially prevents side nailing and they present a major problem in attaching other members to the sides. Also, since the flange portion of the I-joist provides almost all of the spacing and stiffness it cannot be notched as is commonly done with solid sawn lumber. The nature of the geometry increases shear forces in the web member to higher values than are found in solid products of rectangular cross section.
It is notable in view of the highly heterogeneous nature of the smaller trees now available that the art has not more seriously heretofore addressed the problem of producing strong wide and/or long members of uniform and dependable properties from smaller plantation trees. The present invention overcomes the noted deficiencies in solid sawn lumber and composite I-beams. In addition, it results in a much higher utilization of the tree into useful lumber products.